This invention relates to an improved method for nondestructively detecting even very small flaws in a structure. More specifically, it relates to the application of certain phenomena and relationships of fracture mechanics to the generation, detection and interpretation of thermal emission signals emitted by the material comprising the structure undergoing testing.
Almost all structural failures are caused by fatigue, brittle fracture, plastic failure, creep, instability or corrosion. Approximately 90% of all service failures are caused by fatigue flaws.
Fatigue is typically defined as a failure engendered by stress variation attending cyclical loading of the structure. In practice the stress variation may be periodic as with rotating structure such as turbine blades or aperiodic as in the case of an aircraft wing. Structures such as turbine blades may be designed with great fatigue life having, for example, a service life as high as 10.sup.12 stress cycles. Other structures such as glass-filament wound rocket motor cases are designed to experience fatigue failure if exposed to a relatively few stress cycles.
Fatigue strength is often expressed in terms of S-N curves which are coordinate plots of stress amplitude (S) and number of cycles to failure (N). More materials such as steel commonly exhibit an endurance limit. If stress amplitude is maintained below the limit, fatigue failure will not occur. For other types of materials, the test data thus far available do not indicate that an endurance limit exists. For many applications the expected use involves far fewer than 10.sup.8 cycles. The engineers then treat the stress amplitude at 10.sup.8 cycles as a nominal endurance limit.
Whereas in many materials fracture under static loading conditions is generally preceded by readily detectable bulk plastic deformation, no such obvious indications usually signal impending fatigue fracture. Typically a microscopic flaw or crack is formed at a localized point of stress concentration. Under cyclic load the flaw becomes a fatigue flaw. Once formed, the fatigue flaw grows with each continued load cycle. Ultimately a point is reached at which unstable growth occurs, thereby resulting in catastrophic fracture.
Fatigue strength is a complex function of many variables including a spectrum of stress amplitudes, corresponding number of stress-cycles, nature of the applied stress, type of structure and surface conditions.
Almost invariably, the local microscopic stress concentration points at which the onset of fatigue occurs are produced at material, design, manufacturing or corrosion induced discontinuities, such as welds, cutouts, rivet holes, notches, or at voids, scratches, tool marks or other defects.
One type of flaw detection technique which depends upon cyclic loading is acoustic emission. The measured signal depends upon the tearing sounds associated with flaw growth. However, by the time a flaw has propagated sufficiently for detection, the useful life of the structure being tested may have been materially shortened. Thus, in many high reliability applications the effect of flaw propagation in conjunction with structural testing is clearly undesirable.
Illustrative examples of high reliability applications where the effect of flaw propagation in conjunction with structural testing is undesirable may be found in the aerospace industry. One such important area pertains to the ability to determine, as part of a Quality Control incoming inspection procedure, whether certain expensive high reliability structural components, such as jet engine blades and hydraulic tubing, are flawed prior to installation in the aircraft. The ability to detect flaws prior to installation enhances the operational reliability of the aircraft and avoids the unnecessary expense associated with the installation and subsequent removal of the flawed components. Similarly, the ability to accurately predict residual life, particularly where an aircraft or engine component might have reached its specified life, but may not have endured the loading it was designed to survive, is an important area for application of nondestructive testing methods.
It is apparent that one of the limitations of the acoustic emission flaw detection technique is its inherent dependency upon flaw growth. In the testing of high reliability component structures this is clearly undesirable, since the resulting flaw propagation may appreciably shorten the useful life of the structure being tested.
Accordingly, it is an object of the invention to provide an improved method for nondestructively detecting even very small flaws in a structure. More specifically, it is an object of the invention to overcome the aforementioned limitations associated with the acoustic emission flaw detection technique.
It is a further object of the invention to provide a nondestructive thermal emission flaw detection method.